Method and apparatus for the solution deposition of high quality oxide material

ABSTRACT

A metal and oxygen material such as a transparent electrically conductive oxide material is electro deposited onto a substrate in a solution deposition process. Process parameters are controlled so as to result in the deposition of a high quality layer of material which is suitable for use in a back reflector structure of a high efficiency photovoltaic device. The deposition may be carried out in conjunction with a masking member which operates to restrict the deposition of the metal and oxygen material to specific portions of the substrate. In particular instances the deposition may be implemented in a continuous, roll-to-roll process. Further disclosed are semiconductor devices and components of semiconductor devices made by the present process, as well as apparatus for carrying out the process.

STATEMENT OF GOVERNMENT INTEREST

This invention was made, at least in part, under U.S. Government,Department of Energy, Contract No. DE-FC36-07G017053. The Government mayhave rights in this invention.

FIELD OF THE INVENTION

This invention relates generally to the electro deposition oftransparent, electrically conductive oxide materials and in particularto the deposition of transparent, electrically conductive metal oxidematerials.

BACKGROUND OF THE INVENTION

A number of electronic devices incorporate one or more layers oftransparent, electrically conductive material. Such devices include, butare not limited to, semiconductor devices such as electronic memory,photovoltaic devices, photo sensors, other photo responsive devices,display devices and the like. These layers are typically fabricated fromtransparent, electrically conductive metal oxide (TCO) materials; and,zinc oxide based materials comprise one particular TCO material.Transparent, electrically conductive zinc oxide materials are often notstoichiometrically pure, but typically incorporate species such assuboxides, hydroxides, ionic species, dopants and the like which canfunction to enhance electrical conductivity of the electronic device.Therefore, within the context of this disclosure, it is to be understoodthat “metal and oxygen materials” are meant to include materials basedthereon and may also include suboxides, hydroxides, and other species.For example, materials based on zinc and oxygen (sometimes referred toas “zinc oxide” or “zinc oxide material”) may also include suboxides ofzinc, hydroxides of zinc such as Zn(OH)₂, Zn²⁺ ions (typically in theform of zinc salts) and other such species. Likewise other metal andoxygen materials, such as tin and indium based material, may includeoxides, suboxides, hydroxides and ionic species. It is also to beunderstood that in the context of this disclosure, the metal and oxygenmaterials may also include dopants or modifiers such as boron, which canfunction to tailor the electrical conductivity of the deposited oxidematerial (e.g. ZnO) layer and/or control the physical morphology of thedeposited layer.

Zinc oxide materials are one metal and oxygen material which hassignificant utility as components of the back reflector structure ofhigh efficiency photovoltaic devices and the present invention will beexplained with reference to such materials; however, it is to beunderstood that the principles of this invention are applicable to thedeposition of other metal and oxygen materials. The back reflector is animportant component of such devices. It is disposed at the back surfaceof the photovoltaic device, typically as a portion of the supportsubstrate, and functions to reflect and redirect unabsorbed photonswhich have passed through the overlying, photovoltaically activesemiconductor layers back through those layers for reabsorption. Atypical back reflector structure includes a highly reflective metallayer such as a layer of silver or aluminum having a microtextured layerof transparent, electrically conductive zinc oxide material disposedthereatop. The textured nature of the zinc oxide material serves toscatter the reflected photons of incident light that were not absorbedon the initial pass through the superposed solar cell material therebyallowing for their subsequent absorption in their secondary pass throughsaid solar cell.

In order to maximize the efficiency of the photovoltaic device, theelectronic, optical and physical properties of the zinc oxide materialmust be carefully controlled. The zinc oxide material must have goodelectrical conductivity, since photo current generated by the overlyingsemiconductor layers must pass through the zinc oxide material forcollection in the subjacent substrate electrode. Hence, the electricalresistivity of the oxide material represents a parasitic loss in thephotovoltaic device. Likewise, the material must have good opticaltransparency, since reflected photons may pass through the layernumerous times (depending upon the absorption characteristics of thesemiconductor material of the photovoltaic device and the scatteringcharacteristics of the zinc oxide and back reflector layers), and anyoptical absorption will also represent a loss in device efficiency.Finally, the microtexture of the layer needs to be controlled so as tooptimize the scattering of the reflected photons so as to maximize theopportunity of those photons to be absorbed by the overlyingsemiconductor layers. Therefore, the controllable deposition of highquality zinc oxide materials is important to the preparation of highefficiency photovoltaic devices.

The prior art has generally utilized vacuum deposition processes, suchas sputtering, for the deposition of zinc oxide materials. However, suchprocesses are inherently equipment intensive and relatively slowdeposition rates coupled with high capital expenditure costs and highoperational expenses adversely impact the cost of producing photovoltaicdevices. In addition, such deposition processes are inherently slow andrepresent a bottleneck in the photovoltaic device deposition process.Therefore, if high volume deposition processes are to be attempted, theback reflector fabrication stations must be extremely large andexpensive.

Because of the problems associated with the vacuum deposition of suchmaterials, the prior art has attempted to deposit zinc oxide materialsby high speed, low cost electro deposition processes wherein zinc oxidematerials are electroplated onto substrates in an aqueous bath. Somesuch processes are disclosed, for example, in U.S. Pat. Nos. 6,133,061;6,224,736; 6,238,808; and 6,379,521. Despite various attempts, the priorart has not, heretofore, been able to reliably and repeatedly electrodeposit zinc oxide materials having electrical, optical and physicalproperties which maximize their utility in back reflector structures ofhigh efficiency photovoltaic devices. Furthermore, prior art processeshave encountered problems of compatibility when such materials weredeposited on particular substrates.

As will be explained in detail hereinbelow, the present inventionprovides a method and apparatus whereby high quality zinc oxide andother transparent conductive oxide materials may be electro depositedonto a variety of substrates of the type utilized in high efficiencyphotovoltaic devices. Furthermore, the present invention provides amethod and apparatus whereby the deposition of the zinc oxide and othertransparent conductive oxide materials may be limited to preselectedportions of the substrate. Finally, the present invention provides amethod and apparatus which is compatible with the high speed,roll-to-roll fabrication of large area, high efficiency, photovoltaicdevices. These and other advantages of the present invention will beapparent from the drawings, description, and discussion which follow.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is directed to a method for electro depositing alayer of a metal and oxygen material, such as a zinc and oxygenmaterial, onto a substrate. In a first aspect of the present invention,the metal and oxygen material is electroplated onto a substrate in aprocess wherein a first portion of the thickness of the layer isdeposited on the substrate at a first deposition rate, and thereafter asecond portion of the thickness of the layer is deposited atop the firstportion of the thickness at a second deposition rate which differs fromthe first deposition rate. In a specific instance, the second depositionrate is slower than the first deposition rate.

In another aspect of the present invention, a metal oxygen material iselectro deposited onto a substrate in a process wherein at least aportion of the substrate is covered with a masking member which preventsthe deposition of the metal and oxygen material onto those portions ofthe substrate to which it is affixed. The masking member may, in someinstances, be magnetically affixable to the substrate. In specificinstances, the electro deposition process is carried out on an elongatedweb of substrate material which is continuously advanced through adeposition system which includes a deposition station wherein the metaland oxygen material is deposited on the substrate. In this embodiment ofthe invention, a belt-like body of masking material is brought intocontact with a back surface of the substrate member while it is in thedeposition station and while the metal and oxygen material is beingdeposited onto the front surface of the web of substrate material. Insome specific instances, the deposition system may include a biasingmember such as a platen or series of rollers which urge the belt ofmasking material into contact with the substrate.

In yet another aspect of the present invention, the substrate member ismaintained in a partiphobic orientation while the metal and oxygenmaterial is being deposited thereonto so as to at least partiallyinhibit the incorporation of particulate material into the depositinglayer of metal and oxygen material.

In another aspect of the present invention, a layer of metal and oxygenmaterial is electroplated onto a substrate which is disposed in anelectrolyte in a spaced apart relationship with an electrode. In thisprocess, a power supply is operative, when energized, to establish aflow of electrical current through the electrode, the electrolyte andthe substrate so as to deposit a layer of metal and oxygen material onthe substrate. In this process, at least two of the following steps areimplemented: inputting ultrasonic energy into the electrolyte during atleast a portion of the time while the layer of metal and oxygen materialis being deposited onto the substrate; periodically interrupting theflow of electrical current between the electrode, the electrolyte andthe substrate while the layer of metal and oxygen material is beingdeposited; maintaining the substrate in a partiphobic orientation whilethe layer of metal and oxygen material is being deposited thereupon;bubbling a gas through the electrolyte; and energizing the power supplyat a first level while a first portion of the metal and oxygen materialis being deposited on the substrate so that the first portion isdeposited at a first deposition rate, and thereafter energizing thepower supply at a second level during the time that a second portion ofthe layer is being deposited atop the first portion so that the secondportion is deposited at a second deposition rate. In a specificinstance, the second deposition rate is less than the first depositionrate. In some particular instances, at least three of the foregoingsteps are implemented. In further embodiments of this aspect of theinvention, at least one more step from the following group isimplemented: monitoring the composition of the electrolyte bath;monitoring the level of a dopant in the deposited metal and oxygenmaterial; utilizing a dimensionally stable electrode; utilizing anelectrode configured as a hollow basket having particles of the metalcontained therein and utilizing a filter shielded electrode.

The present invention may be implemented in a continuous process, and inspecific instances may be utilized to fabricate back reflectorstructures for high efficiency photovoltaic devices.

The present invention also includes substrates having metal and oxygenmaterials deposited thereupon in accord with the foregoing. Thesubstrates of the present invention may be used as back reflectorstructures for photovoltaic devices. In specific instances, the presentinvention is directed to substrates which include a layer of a highlyreflective metal such as aluminum or silver disposed thereupon andhaving a highly adherent metal and oxygen layer, such as a zinc andoxygen layer, electro deposited thereupon wherein these substrates arecharacterized in that they do not include any vacuum deposited seedlayer of a metal and oxygen material thereupon so that all of the metaland oxygen material deposited upon the reflective metal is depositedfrom a solution in an electro deposition process in accord with thepresent invention.

The present invention is also directed to apparatus for carrying out theaforedescribed methods and for manufacturing the aforedescribedarticles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a photovoltaic device showing a backreflector structure which includes a zinc oxide material deposited inaccord with the present invention;

FIG. 2 is a cross-sectional view of a schematic electroplating apparatuswhich may be utilized to carry out the method of the present invention;

FIG. 3 is a flowchart depicting one embodiment of the present invention;

FIG. 4 is a schematic depiction of an apparatus for implementing themethod of the present invention in a continuous process;

FIG. 5 is an enlarged view of a portion of a deposition station of theapparatus of FIG. 4 better illustrating the masking system; and

FIG. 6 is a depiction of a deposition station generally similar to thatof FIG. 5 but including a biasing platen.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described with reference to the depositionof metal oxides such as zinc oxide materials in connection with thefabrication of back reflector structures for high efficiencyphotovoltaic devices. However, it is to be understood that theprinciples of the present invention may be readily extended to anyapplication wherein high quality metal oxide materials are electrodeposited in a high speed, high volume process. As noted above, suchapplications may include the fabrication of display devices, sensordevices, light emitting devices, and the like.

Referring now to FIG. 1, there is shown a cross-sectional view of ageneralized high efficiency photovoltaic device 10. The deviceincorporates a substrate 12 which functions to support the remainder ofthe device and operates to provide a bottom, current collecting,electrode for the device. In the illustration, the substrate 12 iscomprised of two separate layers. The first layer 14 is a body ofstainless steel. Disposed thereatop is a relatively thin layer of ahighly reflective metal 16, such as aluminum or silver. In otherembodiments of photovoltaic device, the substrate may be comprised of abody of electrically insulating material such as a polymer, glass,ceramic or the like, provided that a layer of electrically conductivematerial is disposed thereupon.

Disposed atop the substrate 12 is a layer of transparent, electricallyconductive metal oxide material, in an exemplary embodiment a zinc oxidematerial, 18. As noted above, this layer is primarily comprised of ZnO,but may further include other zinc based species as well as dopants andthe like. The material comprising the zinc oxide layer 18 is at leastpartially crystalline and as such the surface of this layer may have atexture corresponding to the crystalline features of the material. Ingeneral, it is preferable that the crystalline features have a sizerange of approximately 200-1000 nanometers so as to maximize thescattering of visible light therefrom. The layer 18 has good electricalconductivity and good optical transparency.

Disposed atop the zinc oxide layer 18 is a body of photovoltaicsemiconductor material 20. The active semiconductor layers of this body20 operate to absorb incident photons and create carrier pairs which arecollected by the electrodes of the device. As is known in the art, thisbody 20 may be comprised of a number of layers of semiconductormaterials disposed in various configurations. In one particularembodiment, the semiconductor body 20 is comprised of hydrogenatedsilicon alloy materials, and as such may comprise one or more stackedtriads, each triad comprised of a layer of substantially intrinsicsemiconductor material interposed between p-doped and n-dopedsemiconductor layers.

Disposed atop the photovoltaic body 20 is a top electrode layer 22,which in the instance of this particular configuration of device, isfabricated from an optically transparent, electrically conductivematerial such as ZnO or another TCO material. As is known in the art,current collecting structures such as bus bars, grids and the like maybe disposed upon the top electrode 22.

In the operation of the photovoltaic device, photons pass into thedevice through the top electrode layer 22 and are absorbed by thephotovoltaic body 20 wherein they generate electron-hole pairs. Theinherent, built-in electric field of the photovoltaic body 20 separatesthe photogenerated holes and electrons of these carrier pairs and theyare collected by the respective top electrode 22 and substrate 12.Photons which are not absorbed by the photovoltaic body 20 pass throughthe zinc oxide layer 18 and are reflected by the reflective layer 16.The textured nature of the zinc oxide layer 18 scatters the reflectedphotons so that their angulated path back through the photovoltaic body20 is increased as compared to non-scattered photons. And in someembodiments, the reflective layer 16 will also include a texturedconfiguration to also aid in scattering the reflected photons.

Referring now to FIG. 2, there is shown a generalized system 30 as maybe employed for the deposition of zinc oxide materials in accord withthe present invention. The system 30 includes a tank 32 which isconfigured and operable to retain a volume of electrolyte material 34therein. The apparatus further includes an electrode station having adeposition electrode 36 supported therein. As shown in FIG. 2, theelectrode 36 is configured as a plate, comprised primarily of a metallicmaterial such as zinc metal. It is to be understood that the apparatusof FIG. 2 is generalized, and in some instances the electrode may beconfigured as a mesh, and/or as a nonplanar body. In one embodiment, theelectrode is a hollow, basket-like, perforated body comprised of amaterial which is inert to the deposition process, such as Ti, Pt, Pd,Au, or the like. Zinc particles in the form of shot or the like aredisposed in the hollow body. In another embodiment, a filter ispositioned about the electrode to shield the electrode and preventparticulate matter from reaching the surface of the substrate upon whichthe deposition is taking place. In one embodiment the filter is in theform of a porous, polyethylene filter bag, disposed so as to surroundthe electrode. In another embodiment, the electrode is an inert,dimensionally stable electrode fabricated from an inert material such astitanium. As is known in the art, in electroplating processes of thistype, all of the metal ions which form the deposited metal and oxygenlayers are provided from the electrolyte. As is further to beunderstood, the electrode station may also include fixturing memberssuch as clamps, brackets and the like for supporting the electrode body.Also, as will be further discussed hereinbelow, in some instances theelectrode station may include a plurality of discrete electrodes.

The system of FIG. 2 supports a substrate 38 in the body of electrolytematerial 34. As described above, the substrate 38 may comprise a singlelayered structure or a multilayered structure.

The electrode 36 and the substrate 38 are both in electricalcommunication with a power supply station which includes power supply 40which in turn is controlled by a controller 42. The power supply 40 is aDC power supply, and the electrode 36 is in communication with thepositive terminal of the power supply 40 and the substrate is inelectrical communication with the negative terminal of the power supply40. The illustrated embodiment of FIG. 2 includes a single power supply40; however, it is to be understood that in other embodiments, the powersupply station may include a number of power supplies operative toenergize a plurality of discrete electrodes and/or to provide differentlevels of power.

As is further illustrated, the system 30 includes a heater 44 disposedin the tank 32. The heater 44 is operative to maintain the electrolyte34 at a preselected temperature, and in that regard, the heater 44 has acontroller 46 associated therewith. As illustrated herein, the heater 44is an electrical resistance heater; although, other types of heater asis known in the art may be likewise utilized.

The system 30 also preferably includes a gas bubbler 48 disposed in thetank. The bubbler 48 has a gas supply 50 associated therewith and isoperable, when activated, to bubble a gas, such as air or nitrogen,through the electrolyte 34, so as to keep the electrolyte stirred.

The system further includes an ultrasonic transducer 52 disposed in thetank. The transducer is energized by a controller 54 and is operative,when energized, to introduce ultrasonic energy into the electrolytematerial 34. While not wishing to be bound by speculation, the inventorshereof presume that the ultrasonic energy may act to maintain thecleanliness of the surface of the deposition substrate and/or thecleanliness of the depositing layer by removing unwanted speciestherefrom.

The systems of the present invention may further include a monitoringstation for measuring the composition of the electrolyte during thedeposition process, so as to determine the concentration of metal ions,dopants and other species. Such monitoring is preferably done in situand in real time, and assures the uniformity and consistency of thedeposited materials. Monitoring may be by techniques includingpotentiometric techniques, chemical techniques such as EDTA titration,spectroscopic techniques and the like. Monitoring can be utilized incombination with reagent supply systems operating in a feedback mode.Thus, for example, if the metal concentration of the electrolyte is toolow, additional metal can be added. Or, if the pH is too high, acid canbe automatically added. Likewise, the system can control and adjustdopant reagent levels based upon measured levels in the electrolyteand/or the deposited layer.

In FIG. 2, the substrate material 38 is shown as having a body ofmasking material 56 affixed to one surface thereof. The masking materialoperates to shield portions of the substrate so that in the process,zinc oxide material is unable to be deposited onto those shieldedportions of the substrate. This feature is optional in the practice ofthe present invention; however, in a number of processes and deviceconfigurations it has been found beneficial to so restrict the depositof the zinc oxide material. The masking material may be variouslyconfigured and adhered to the substrate and as such may comprise apolymeric resist coating. However, in one specific embodiment of thepresent invention, the masking material 56 comprises a sheet of materialwhich is magnetically affixable to at least a portion of one surface ofthe substrate. In this regard, the masking material 56 may comprise asheet of magnetized metal, or it may comprise a body of polymericmaterial having magnetized particles dispersed therein. In specificinstances, the masking material is electrically insulating, so as topreclude deposition thereonto.

In a typical process for the deposition of zinc oxide material in accordwith the present invention, the electrolyte material 34 comprises anapproximately 0.03 molar solution of Zn(NO₃)₂. In some embodiments, theelectrolyte will also include relatively small amounts of adhesionpromoting material such as ethylenediaminetetraacetic acid (EDTA). Otherchelating materials and/or adhesion promoters such as fumaric acid,malic acid, various other compounds having multiple functional groups,as well as compounds such as sucrose may likewise be included.Typically, the concentration of these materials is in the range of 1-200ppm. The electrolyte material may also include one or more dopant ormodifying species which operate to enhance the electrical conductivityof the deposited zinc oxide material. One specific doping speciesutilized in the present invention comprises boron, and it may be presentin the electrolyte in the form of boric acid at a concentration in therange of 0.1%-1.0% by weight. The electrolyte is generally maintained ata temperature in the range of 50-100° C. during the deposition process,and in a typical instance, the electrolyte is maintained at atemperature of approximately 80° C.

In those instances where tin and oxygen based materials are beingdeposited, the electrolyte will include one or more tin salts such astin chloride, tin acetate, tin sulfate, and the like. The deposition ofindium based materials will employ an electrolyte which includes indiumsalts such as indium chloride, indium nitrate, indium sulfate and thelike.

The power supply is activated so as to establish an electrical potentialof approximately 0.5 to 20 volts between the electrode 36 and thesubstrate 38. This potential will cause the deposition of zinc oxidematerial onto the substrate, and the rate of deposition will beproportional to the power density at the substrate. Therefore, thecontrol of deposition power will allow for the control of the depositionrate. In a typical deposition, power density at the substrate will be inthe range of 0.5-20 mA/cm².

In order to enhance the uniformity of the deposited zinc oxide, theelectrolyte bath 34 is at least periodically stirred, and this may bedone by use of a recirculation pump (not shown) and/or by bubbling a gasthrough the electrolyte from the bubbler 48. It has been found, in thisprocess, that air or nitrogen may be employed for this purpose; however,other gases which are inert or do not otherwise degrade the depositionprocess may likewise be employed.

In accord with another aspect of the present invention, it has beenfound that the quality of the deposited zinc oxide material is improvedif ultrasonic energy is at least periodically introduced into theelectrolyte bath. In one embodiment, the ultrasonic transducer 52 isenergized at a power level of approximately 500 watts, for example. Theconfiguration of the ultrasonic energy system employed will depend onthe configuration of the electrical device and other aspects of theelectro deposition system.

In another aspect of the present invention, it has been foundadvantageous to operate the power supply 40 in a pulsed mode wherein theDC current applied to the electrode 36 and substrate 38 is periodicallyinterrupted. In a typical process, the current is pulsed at a rate of 1to 10 Hz. While not wishing to be bound by speculation, Applicantpresumes that operation in the pulsed mode allows for equilibration ofdeposition conditions at the surface of the substrate and therebypromotes the deposition of materials having optimum compositions andmorphology.

In accord with yet a further aspect of the present invention, theinventors hereof have found that very high quality deposits of zincoxide material may be prepared in a multi-deposition rate process. Inthis embodiment of the present invention, the substrate is initiallycoated with a first layer of zinc oxide material in a relatively highrate deposition process. High rate deposition may be achieved bycontrolling the power supply so as to energize the electrode 36 andsubstrate 38 with a relatively high level of power. This produces arelatively fast deposition of a relatively thick portion of the body ofzinc oxide material. Thereafter, the power supply energizes theelectrode 36 and substrate 38 at a lower level of power so as to depositzinc oxide material upon the previously deposited layer, at a lowerrate. It is believed that this lower rate material manifests a very goodcrystalline structure which optimizes the performance of the zinc oxidelayer. Use of the dual rate process thus achieves the benefits of highaverage deposition rate while producing a body of zinc oxide materialhaving superior electrical, optical and physical properties. In furtherrefinements of this process, the body may be deposited at three or moredeposition rates Also, it is to be noted that the change in depositionrate need not be abrupt, and within the context of this aspect of theinvention, the deposition rate may be varied on a continuous basis, byvarying current density, so that the material transitions from high rateto low rate, or from a low rate to a high rate, in a non-stepwise, oronly partially stepwise manner.

In yet another aspect of the present invention, it has been found thatsuperior quality materials are prepared when the substrate 38 ismaintained in an orientation which will allow gravity to inhibit theaccumulation of particulate matter thereupon. As such, the substrate 38may be oriented vertically as is shown in FIG. 2. However, otherorientations which will inhibit particle accumulations may be employed.For example, the substrate may be disposed in a horizontal orientationwith the deposition surface facing downward. In other instances, thesubstrate may be disposed in an angled relationship with a verticalaxis, provided that the deposition surface is downwardly inclined so asto inhibit particulate accumulation. Within the context of thisdisclosure, all of such orientations of the substrate, wherein gravityacts (at least in part) to inhibit particle accumulation on thedeposition surface, are referred to as “partiphobic”.

The zinc oxygen materials produced by the present invention have verygood physical, optical and electronic properties which make them ideallysuited for use in back reflector structures of photovoltaic devices. Itis believed that this combination of properties is resultant from theindependent and/or synergistic effect of at least two and perhaps moreof the aforedescribed features of the present invention, namely the useof pulsed power, deposition of the material in an at least dual layeredstructure at differing power levels, ultrasonic cleaning of thedepositing layer during the deposition process, and use of a partiphobicsubstrate orientation which precludes particulate inclusions. Otherfactors which can contribute to the quality of the materials produced bythe present process include the use of in situ monitoring of electrolytebath composition; in situ monitoring of dopant composition and profiles;and the use of electrode structures such as the hollow basket,dimensionally stable electrode and/or filter shielded electrodepreviously discussed. Typical layer thicknesses in back reflectorstructures are on the order of 0.1 to 3 microns, and the high speednature of the deposition process of the present invention greatlyenhances the economics and physical implementation of the fabricationprocess as compared to methods wherein the layer is entirely depositedby vacuum processes.

While the present invention provides for the high speed electrochemicaldeposition of zinc oxide materials, it is to be understood that in someinstances, the invention may be implemented in connection with anoverall fabrication process wherein some portions of the zinc oxidematerial may be deposited in a vacuum process such as sputtering. Forexample, commonly employed substrates for photovoltaic devices comprisestainless steel having a reflective coating of silver or aluminumdeposited thereupon. The reflective layer is fairly thin and is oftendeposited by sputtering or some other vacuum process. In some instances,it has been found advantageous to vacuum deposit a relatively thin“seed” layer of zinc oxide material atop the reflective layer. Thisdeposition is typically carried out by sputtering, and the total layerthickness is on the order of 5-100 nanometers; consequently, depositiontime is relatively fast. It has been found that in some instances, theuse of the vacuum deposited seed layer facilitates the deposition andadhesion of the electrochemically deposited zinc oxide material. Itshould be noted that the use of a seed layer is optional, and in accordwith the present invention, the inventors herein have been able toelectro deposit a high quality TCO material having very good adhesionproperties and device operational parameters atop various reflectivesubstrates without a seed layer, thereby reducing manufacturing costsconsiderably. Elimination of the seed layer is particularly important inthose instances where the reflective layer is deposited byelectroplating, since this allows for a total atmospheric pressureprocess. It has been found that the inclusion of adhesion promoters suchas EDTA in the electrolyte enhances the adhesion of the electrodeposited layer to the reflective metal, and thereby eliminates the needfor a seed layer. In an experimental series it was found that theadhesion of zinc oxide layers directly electro deposited onto silverlayers from an EDTA containing bath was at least as good as that ofcomparable layers of zinc oxide electro deposited onto a silver layerhaving a vacuum coated seed layer of zinc oxide thereupon. If theadhesion promoter is eliminated from the bath, adhesion of the zincoxide layer is poor in the absence of the seed layer. In thisexperimental series, adhesion was measured by the tape lift-off method.

In one specific implementation of the present invention, the substrateis approximately 5 mils thick layer of stainless steel. In thoseinstances where a reflective layer is to be sputtered thereatop, anapproximately 100 nanometer thick adhesion layer of titanium is vacuumdeposited upon the stainless steel. Subsequently, a reflective layer ofsilver or aluminum, having a thickness in the range of 100-500nanometers is deposited upon the substrate. Thereafter, a seed layer ofzinc oxygen material having a thickness of approximately 40 nanometersis deposited atop the reflective layer. The thus prepared substrate iscoated with a layer of zinc oxide material in the process of the presentinvention. The thickness of this layer is generally in the range of0.1-3 microns depending upon specific applications.

Referring now to FIG. 3, there is shown a generalized flowchartdepicting one embodiment of the present invention. As is shown in FIG.3, the process employs a substrate which, as mentioned above, mayoptionally include a seed layer thereupon. In a first portion of thedeposition process, the zinc oxide material is deposited onto thesubstrate at a relatively high deposition rate, which in some instancesis approximately 10 nm/sec. This initial deposition is carried out at atemperature in the range of 50-100° C., and typically at a temperatureof 80° C. The electrolyte in the deposition tank is agitated byactivating the gas bubbler system; however, agitation may optionally becarried out by pumps, stirrers or the like. After a portion of the layer(typically 30-80; and in specific instances 50-70% of its thickness) hasbeen deposited, ultrasonic energy is input to the deposition tank.Deposition conditions are maintained at a high rate, and agitation ofthe bath is also continued. The ultrasonic energy serves to removeundesirable solution particulates from the depositing layer. Any pittingleft by the removal of the loosely adherent materials is filled in bythe depositing zinc oxide material. In this second stage of the process,the remainder of the thickness of the final zinc oxide layer isdeposited.

In the third stage of the deposition process, a further portion of thelayer of zinc oxide material is deposited at a relatively low depositionrate. In particular instances, this rate is in the range ofapproximately 1-5 nm/sec. The deposition bath is maintained atapproximately the same temperature it was in the first two stages, andagitation of the electrolyte is maintained through the use of thebubbler or other means.

Other modes of deposition may be employed. In one instance, the initialdeposition may be at a low rate, followed by high rate deposition; andoptionally followed by a second low rate deposition. In general, it isbelieved that low rate deposition promotes the formation of a layerhaving larger crystals which operate to promote optimum lightscattering. Also, the low rate material can provide good adhesion tosubjacent layers. In addition, low rate material can provide a templatefor subsequently deposited high rate material so that the crystallinestructure of the high rate material resembles that of the low ratematerial to some degree.

Once the total thickness of the layer of zinc oxide material isdeposited, the substrate is then rinsed with water and dried. Drying istypically carried out utilizing atmospheric air either in an oven orthrough the use of a blower. Drying is generally carried out at elevatedtemperatures, typically in the range of 25-200° C. for times ofapproximately 2 minutes. The drying step serves to remove water, butalso allows for the at least partial conversion of zinc hydroxidespecies into zinc oxide species. The drying also can function to annealthe material, thereby further increasing its adherence to the substrate.In one particular instance, the drying/annealing is at 120° C. for twominutes; in another, it is at 150° C. for 1 minute. Following thedrying/annealing, the process is complete, and the substrate may besubsequently processed into photovoltaic devices.

The process of the present invention may be readily implemented in acontinuous, roll-to-roll process for the preparation of photovoltaicsubstrate material, and one such implementation is shown in FIG. 4.Depicted therein is a roll-to-roll deposition apparatus 60 for thecoating of an elongated substrate web with a zinc/oxygen material. Thesystem 60 of FIG. 4 includes a payoff station 62 which supports andfeeds out a web of substrate material 38 from a supply roll 64. As isknown in the art, the payoff station may include turning rollers,steering rollers, a tensioning mechanism and the like.

The system 60 includes three deposition stations 66, 68 and 70, althoughit is to be understood that in other implementations, greater or lessernumbers of deposition stations may be employed. In this particularimplementation, the stations 66, 68 and 70 are configured to carry outthe three stages of the deposition as described with reference to FIG.3. As such, the first station 66 carries out a relatively high speeddeposition wherein the electrolyte material is agitated by the bubbler48. In the second station 68, high speed deposition is carried oututilizing bubbler agitation as well as ultrasonic energy input from theultrasonic transducer 52. The third deposition station 70 is used forthe low rate deposition. It also includes a bubbler 48 for maintainingagitation of the electrolyte.

Each of the stations includes a heater 44, and it is notable that inthis embodiment, each deposition station 66, 68 and 70 includes twodeposition electrodes. In this regard, the first station includeselectrodes 36 a, 36 b, the second station includes electrodes 36 c, 36 dand the third includes electrodes 36 e, 36 f. Use of dual electrodesspeeds up the deposition process. As described with reference to FIG. 2,the electrodes 36 are all in communication with an appropriate powersupply and energized at power levels sufficient to provide a desireddeposition rate.

As discussed above, it is frequently desirable to include a body ofmasking material which operates to prevent deposition of the zinc/oxygenmaterial onto particular portions of the substrate. In the illustratedembodiment, each deposition station includes a masking system which asillustrated is comprised of two portions 72 a, 72 b.

Referring now to FIG. 5, there is shown an enlarged view of a portion ofthe first deposition station of FIG. 4, better illustrating the maskingsystem. As depicted, a portion of the substrate web 38 is advanced pasta first deposition electrode 36 a, about a turning roller 76, and past asecond deposition electrode 36 b. The first masking system 72 a isdisposed so as to contact the back surface of the substrate 38 with abody of masking material 78, when it is in the region of the firstelectrode 36 a. The masking material 78 is flexible, electricallyinsulating and magnetic, and as such may comprise a polymer having amagnetic substance embedded therein. The masking material 78 isconfigured as a continuous web, and it is supported by a first 80 and asecond 82 roller. In the operation of the system, the web 38 advancesthrough the deposition station and is contacted by the magnetic material78 which adheres thereto. The web of magnetic material 78 travels alongwith the substrate, past the electrode 36 a. The magnetic nature of themasking material maintains it in contact with the substrate. After thesubstrate 38 leaves the region of the first electrode, the second roller82 pulls the masking material 78 away from the substrate 38. The secondmasking system 72 b is disposed in association with the second electrode36 b and operates in a similar manner to the first masking system 72 a.

The substrate masking system may be configured to include rollers,platens, and the like which can assist in biasing the masking memberagainst the substrate. These biasing systems may be used in combinationwith a magnetically affixable masking member; although, in someinstances, the biasing force may be sufficient to assure good contactbetween the substrate and the biasing member so that magnetic attractionneed not be employed. Referring now to FIG. 6, there is shown oneembodiment of biasing system as configured to be utilized in adeposition station of the type generally shown in FIG. 5; and in thatregard, similar elements will be identified by similar referencenumerals. The deposition station of FIG. 6 includes a first and a seconddeposition electrode 36 a, 36 b disposed and operative to electrodeposit a layer of zinc oxide material onto a web of substrate material38 passing through the deposition station. The deposition station ofFIG. 6 further includes a first masking system 72 a and a second maskingsystem 72 b which, as previously described, include a flexible,electrically insulating body of masking material 78 supported by a first80 and a second 82 roller. The system of FIG. 6 further includes acurved biasing platen 84 which is disposed so as to contact the belt ofmasking material 78 and urge that material against a portion of thesubstrate 38. A second such platen 86 is associated with the secondmasking system 72 b. Biasing may be accomplished by otherwise configuredmembers. For example, the biasing platens 84, 86 may be replaced by oneor more rollers. Since the biasing platens urge the masking materialinto contact with the substrate, the masking material need not bemagnetic, although a magnetic body may be utilized.

Returning now to FIG. 4, it will be seen that the system 60 furtherincludes a rinsing station 84 disposed downstream of the depositionstations 66, 68 and 70. The rinsing station 84 comprises a tankconfigured so that the coated substrate passes therethrough wherein itis rinsed with water. The rinsing station 84 may further includeagitators, stirrers or the like for enhancing the rinse action. It mayalso include a flow-through system for continuously replacing the rinsewater. In some instances, the rinse station may comprise two or morediscrete rinse tanks.

Downstream of rinsing station 84 is a drying station 86 wherein thecoated web is dried as described above. The drying station may comprisean oven, a drying tunnel, or the like and may include radiant heaters,hot air blowers or the like. Following the drying, the substratematerial is then wound onto a take-up reel 88 in a take-up station 90.

The coating of the zinc and oxygen material may also be implemented intoa single, continuous process in which a reflective layer iselectroplated onto a stainless steel web and thereafter coated with thezinc and oxygen material. In this regard the apparatus may include afirst deposition station wherein the substrate is electroplated with areflective layer of silver or aluminum. For example, silver may beelectroplated onto the stainless steel from an electrolyte bathcomprising: 37.5 g/l dimethylhydantoin, 12.0 g/l silver nitrate, 0.38g/l thiamine hydrochloride, 7.5 g/l potassium chloride and 7 g/lpotassium hydroxide. Plating takes place at a temperature of 60-90° C.,using a silver electrode at a current density of about 3 mA/cm² anddeposits a highly reflective silver layer at a rate of about 2 nm/sec.Aluminum may likewise be electroplated by processes known in the art.

The system of FIG. 4 produces an elongated web of substrate materialwhich may subsequently be employed in a continuous process for thefabrication of photovoltaic devices. In that regard, the roll ofmaterial may be transferred to a photovoltaic deposition apparatus. Inyet other instances, the substrate coating system may be placed in linewith, or incorporated into, a photovoltaic deposition apparatus. Forexample, a roll to roll process may be implemented in which a reflectivelayer of silver or aluminum is first electroplated onto the substrate,and thereafter a layer of a zinc and oxygen material is electroplatedonto the substrate in the same apparatus. The coated substrate may thenbe conveyed to a series of semiconductor deposition chambers associatedwith the same apparatus, or it may be subsequently conveyed toseparately disposed semiconductor deposition chambers.

The present invention provides a method and apparatus for the rapid,efficient deposition of high quality layers of metal oxide material,such as zinc oxide material. The invention has been described withregard to particular apparatus and particular operating conditions asspecifically adapted for the preparation of substrates for highefficiency photovoltaic devices. However, it is to be understood thatthe principles of the present invention may be extended to other methodsand apparatus and to processes for the preparation of devices andmaterials other than those for use in photovoltaic applications. Assuch, numerous modifications and variations of the invention will beapparent to those of skill in the art in view of the teaching presentedherein. It is to be understood that the foregoing drawings, discussionand description are illustrative of specific embodiments of theinvention, but are not meant to be limitations upon the practicethereof. It is the following claims, including all equivalents, whichdefine the scope of the invention.

1. In a method for the electroplating of a layer of a metal and oxygenmaterial onto a substrate wherein said substrate is disposed in anelectrolyte in a spaced apart relationship with an electrode, andwherein a power supply is operative, when energized, to establish a flowof electrical current through said electrode, said electrolyte, and saidsubstrate so as to deposit a layer of said metal and oxygen material onsaid substrate, characterized in that said deposition process includesat least two steps from the group consisting of: inputting ultrasonicenergy into said electrolyte during at least a portion of the time whilesaid layer of metal and oxygen material is being deposited onto saidsubstrate; periodically interrupting the flow of electrical currentbetween said electrode, said electrolyte, and said substrate, while saidlayer of said metal and oxygen material is being deposited; maintainingsaid substrate in a partiphobic orientation while said layer of metaland oxygen material is being deposited thereupon; bubbling a gas throughsaid electrolyte; and energizing said power supply at a first levelduring the time that a first portion of said metal and oxygen materialis being deposited on said substrate so that said first portion isdeposited at a first deposition rate and thereafter energizing saidpower supply at a second level during the time that a second portion ofsaid layer is being deposited atop the first portion wherein said secondlevel of power is selected so that the deposition rate of said secondportion is less than the deposition rate of said first portion.
 2. Themethod of claim 1, wherein the improvement comprises implementing saidprocess so as to employ at least three of said steps.
 3. The method ofclaim 1, including at least one further step selected from the groupconsisting of: monitoring the composition of the electrolyte bath;monitoring the level of a dopant in the deposited metal and oxygenmaterial; utilizing a dimensionally stable electrode; utilizing anelectrode configured as a hollow basket having particles of said metalcontained therein; and utilizing a filter-shielded electrode.
 4. Themethod of claim 1, wherein said method is operative to deposit a layerof a zinc and oxygen material onto the substrate.
 5. An apparatus forcarrying out the method of claim
 1. 6. A layer of a zinc and oxygenmaterial made by the method of claim 4.